Fuel cell comprising a magnetic cathode with static pumping

ABSTRACT

The fuel cell, generating electric power from oxygen and hydronium ions and comprising an anode (A), a magnetic cathode, comprising an active layer ( 2 ), and a proton electrolyte ( 1 ) between the anode and the cathode, comprises a network ( 3 ) of permanent magnets ( 4 ) designed to increase the diffusion of oxygen in the active layer. The centers of the magnets ( 4 ) of the network ( 3 ) of permanent magnets are preferably arranged with a two-dimensional distribution in a plane arranged at the interface between the electrolyte ( 1 ) and the active layer ( 2 ), the magnets being magnetized in parallel manner along the axis perpendicular to this plane. In this way, all the poles of one polarity (S) are surrounded by the active layer ( 2 ), all the poles of opposite polarity (N) being surrounded by the electrolyte ( 1 ).

BACKGROUND OF THE INVENTION

The invention relates to a fuel cell generating electric power fromoxygen and hydronium ions, and comprising an anode, a magnetic cathodecomprising an active layer, a proton electrolyte between the anode andthe cathode, and a network of permanent magnets having magnetic axesperpendicular to the interface between the electrolyte and the activelayer, the magnets comprising a first pole and a second pole.

STATE OF THE ART

Fuel cells are constituted by an anode and a cathode separated by aliquid or polymer electrolyte. For certain applications, in particularpower supply of portable electronic devices, one of the fuels is theoxygen of the air. The performances of such a system are limitedessentially by the cathode and in particular by the quantity of oxygenaccessible at the level of the catalyzer. The use of a conventionalpumping system increasing the oxygen flow at the level of the cathode iscostly in energy, the associated performance increase then beingcompensated by the energy consumed by the pumping system.

It would be interesting to make the fuel cell operate by using theoxygen present in the ambient air to the maximum without a mechanicalpumping system. A solution called “static pumping” has been proposed,using the paramagnetic properties of oxygen. Static pumping is based onthe force exerted on a paramagnetic object by a magnetic field in whichit is situated. In a magnetic field this force attracts the paramagneticobject in the direction in which the absolute value of the magneticfield increases.

The article “Magnetic Promotion of Oxygen Reduction Reaction with PtCatalyst in Sulfuric Acid Solutions” by N. I. WAKAYAMA et Al. proposedimproving the operation of a fuel cell by static pumping (Jpn. J. Appl.Phys. Vol. 40 (2001) pp. L269-L271) by incorporating a powder of smallmagnetic particles in an active layer between a membrane and a diffusionelectrode. However, this solution has a very small effect, because themagnetic particles are distributed in random manner over the wholethickness of the active layer.

The document JP 2002/198,057 describes a fuel cell comprising permanentmagnets dispersed in one of the electrodes of a fuel cell. The magnetscan be arranged in a network and the orientations of the permanentmagnets are uniform and parallel to a line connecting the electrodes.

Consequently, in the two above-mentioned documents, the resultingmagnetic force is reduced in the points of the space where the magneticfields of several particles or magnets are opposed. The oxygen is notattracted by the magnetic forces to penetrate into the whole volume ofthe active layer. The operation of the active layer is then improved onthe surface only, whereas the operation within the volume remainsweakened.

Another drawback of small magnetic particles is the large corrosion ofthe magnetic material in an acid or even alkaline medium depending onthe type of cell envisaged.

OBJECT OF THE INVENTION

The object of the invention is to remedy these shortcomings and inparticular to increase the quantity of oxygen accessible at the level ofthe whole of the catalyzer of the active cathodic layer.

According to the invention, this object is achieved by the accompanyingclaims and more particularly by the fact that the first and second polesof the magnets of the network are respectively arranged in an activelayer and in the electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from thefollowing description of particular embodiments of the invention givenas non-restrictive examples only and represented in the accompanyingdrawings, in which:

FIG. 1 is a representation of an embodiment of a fuel cell according tothe invention.

FIG. 2 illustrates the variations of the magnetic force inside the cell.

FIGS. 3 and 4 are cross-sectional views along the vertical axis 8 ofdifferent embodiments of the cell according to FIG. 1.

FIG. 5 schematically represents the symmetry of another particularembodiment of a network of permanent magnets.

DESCRIPTION OF PARTICULAR EMBODIMENTS

FIG. 1 represents a fuel cell comprising an anode A, a protonelectrolyte 1 and a magnetic cathode comprising an active layer 2, aporous electric current collector plate 5 and a diffusion layer 6. Theoxygen arriving from the right passes through the collector plate 5 andthe diffusion layer 6 of the cathode and enters the active layer. Thehydrogen comes in the form of hydronium ions (usually called H⁺), borneby a compound able to be a hydrogen vector (alcohol, sugar, nitrogenatedcompound, etc . . . ).

To increase the diffusion of the oxygen entering the active layer 2, thecathode comprises a network 3 of permanent magnets 4 having magneticaxes perpendicular to the interface between the electrolyte and theactive layer.

In a preferred embodiment, the centers of the magnets 4 of the network 3of permanent magnets are arranged with a two-dimensional distribution.In FIG. 1, this two-dimensional distribution is localized in the planeparallel to the interface between the electrolyte 1 and the active layer2. The magnets 4 are preferably magnetized along the axis zperpendicular to the plane of the two-dimensional distribution so thatall the north polarity poles N are in one plane and all the southpolarity poles S are in a parallel plane. In this way, first poles S ofthe magnets 4 of the network 3 are arranged in a first plane parallel tothe interface between the electrolyte 1 and the active layer 2 andsecond poles N of the magnets 4 of the network 3 are arranged in asecond plane parallel to the interface between the electrolyte 1 and theactive layer 2.

In a preferred embodiment, the permanent magnets 4 are semi-surroundedby the active layer 2 so that all the poles (S) of the same polarity aresurrounded by the active layer 2, all the poles of opposite polarity (N)being surrounded by the electrolyte 1. In this way, the first and secondplanes are respectively arranged in the active layer 2 and in theelectrolyte 1. In the embodiment represented in FIG. 1, the interfacebetween the electrolyte 1 and the active layer 2 is arrangedsubstantially at equal distance from the first and second planes. Thepermanent magnets 4 preferably have identical shapes and identicalspatial orientations, as represented in FIG. 1.

In the embodiment represented in FIG. 1, the interface between theelectrolyte and the active layer is situated on a vertical axis 8 andthe magnets are magnetized along a horizontal axis z. The magnets thencreate a magnetic field, the absolute value whereof is maximal on thevertical axis 8. A magnetic force F(z) attracts the oxygen to thevertical axis 8.

In FIG. 2, the magnetic force F(z) is illustrated as a function of thecoordinate on the horizontal axis z. The force F(z) increases when thevertical axis 8 is approached and changes sign precisely on the verticalaxis 8, corresponding to a change of direction of the force. On the leftpart of the axis 8, the oxygen is then attracted to the right, whereason the right part of the axis 8, the oxygen is then attracted to theleft.

The electrochemical reaction with the oxygen takes place in the entireactive layer 2. This layer therefore has to be located in the regionwhere the oxygen concentration is at the maximum. The oxygen coming fromthe diffusion zone 6 is attracted into the whole volume of the activelayer by the magnets. On the other hand, in the electrolyte, the oxygenis repelled towards the active layer so that the oxygen concentration inthe electrolyte is reduced. Insertion of the magnets partially in theactive layer and partially in the electrolyte is optimized by thedistribution of the magnets 50% in the active layer and 50% in theelectrolyte.

With reference to FIG. 3, the network of permanent magnets can be formedby cylindrical magnets 4 arranged in a two-dimensional distribution of aperiodic network 10.

As represented in FIG. 4, the cell can comprise a support network 11comprising apertures 12 wherein the magnets 4 can be arranged. Thesupport comprises passages 13 for the ions, in particular the hydroniumions coming from the electrolyte, between the magnets. The passages 13are therefore triple point zones where the hydronium ion H⁺, oxygen O₂and electron elements are in presence, which gives rise to theelectrochemical reaction. The material of the support network 11 can bea non-magnetic material. The support network can be fixed onto theelectrolyte 1 or arranged at the interface between the electrolyte 1 andthe active layer 2.

The performance of this improved oxygen diffusion system by a network 3of magnets 4 depends on the variation of several parameters: themagnetization, the geometry and number of magnets 4, the thickness ofthe cathode and the geometric distribution of the magnets 4 and of thepassages 13 for the hydronium ions. In this way, with a flat periodicdistribution of the centers of the masses of the magnets 4, as in FIG.3, a uniform improvement of the gas diffusion in the catalyzer isobtained. Other flat geometries, for example triangular or fractal, canalso be envisaged.

As represented in FIG. 5, a distribution of the apertures 12 for placingthe magnets 4 and of the passages 13 in the support network 11 canconstitute a fractal structure, represented by triangles of differentdimensions, a relatively large triangle being surrounded by smallertriangles. The centers of the triangles of FIG. 5 represent the centersof the magnets. The individual shape of the magnets themselves is notnecessarily triangular.

In order to prevent corrosion of the magnets 4 in the electrolyte 1(acid or alkaline), the magnets 4 can be treated against corrosion orcomprise anti-corrosive coatings (in FIG. 1, one of the magnets isrepresented with an anti-corrosive coating 14). The anti-corrosiontreatment depends on the nature of the electrolyte 1. The material ofthe coating is typically platinum or gold.

The network 3 of permanent magnets 4 can comprise magnets 4 made offerromagnetic material. For example, the permanent magnets 4 can be madefrom materials forming part of the SmCo, AlNiCo, NdFeB or Ferritefamilies. However, any magnetic metals and alloys are envisageable.

The best performances are obtained if the magnets 4 are very close tothe oxygen, i.e. on the cathode side. However, an optimum oxygendiffusion throughout the cathode is obtained with the embodiment of FIG.1 wherein the centers of the magnets 4 are located on the interfacebetween the active layer 2 of the cathode and the electrolyte 1. Themagnetic forces increase quickly when the distance between the magnets 4and the oxygen decreases. In this way, the network 3 of magnets 4operates as a filter of the oxygen in the air, privileging oxygen overthe other gases present in the air.

The permanent magnets 4 constitute an ideal magnetic field source,operating alone, without an additional external energy input.

The invention is more particularly suited to fabrication of mini-fuelcells. The network 3 of magnets 4 enables a sufficient magnetic force tobe produced at a distance of a few millimeters from the active layer 2.This enables a reduction of the overpotential of the oxygen reductionreaction to be obtained, as indicated by the following example: in thecase of a fuel cell comprising a solid polymer electrolyte with acathode with a thickness of about 250 μm and a resulting magnetic fieldof the magnets of 10⁻⁶ teslas, a decrease of the diffusion overpotentialof about 10% to 20% can be forecast.

1.-8. (canceled)
 9. Fuel cell, generating electric power from oxygen andhydronium ions, and comprising an anode, a magnetic cathode comprisingan active layer, a proton electrolyte between the anode and the cathode,and a network of permanent magnets having magnetic axes perpendicular toa plane interface between the electrolyte and the active layer, themagnets comprising a first pole and a second pole, fuel cell wherein thefirst and second poles of the magnets of the network are respectivelyarranged in the active layer and in the electrolyte.
 10. Fuel cellaccording to claim 9, wherein the interface between the electrolyte andthe active layer is arranged substantially at equal distance from thefirst and second poles of the magnets.
 11. Fuel cell according to claim9, comprising a support network, comprising apertures wherein themagnets are arranged, and passages for the hydronium ions and theoxygen.
 12. Fuel cell according to claim 11, wherein the support networkis made of non-magnetic material, fixed onto the electrolyte.
 13. Fuelcell according to claim 9, wherein the magnets comprise ananti-corrosive coating.
 14. Fuel cell according to claim 13, wherein theanti-corrosive coating is made of platinum or gold.
 15. Fuel cellaccording to claim 9, wherein the magnets are distributed in a planeparallel to the interface between the electrolyte and the active layerwith a periodic distribution.
 16. Fuel cell according to claim 9,wherein the magnets are distributed in a plane parallel to the interfacebetween the electrolyte and the active layer with a fractal typedistribution.